Manipulation of Phase and Microstructure at Nanoscale for SiC in

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Manipulation of phase and microstructure at nanoscale for SiC with molten salt synthesis: An unconventional ionothermal route Xiaofeng Liu, Cristina Giordano, and Markus Antonietti Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm303727g • Publication Date (Web): 25 Apr 2013 Downloaded from http://pubs.acs.org on April 30, 2013

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Chemistry of Materials

Manipulation of phase and microstructure at nanoscale for SiC in molten salt synthesis Xiaofeng Liu*, Markus Antonietti, and Cristina Giordano Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Research Campus Golm, 14424 Potsdam, Germany. KEYWORDS: silicon carbide; molten salt synthesis; nanoparticle, phase transition; Rietveld analysis Supporting Information Placeholder ABSTRACT: Silicon carbide (SiC) is a compound with strong covalent bonding, which gives its high mechanical strength and oxidation resistance, but also hinders its synthesis under moderate conditions. Herein a facile route is presented for the synthesis of SiC nanomaterials from simple and abundant raw materials in an inorganic molten salt (MS). With this route we are able to synthesis nanoscale 3C-SiC and 2H-SiC in a controlled manner, where the choice of cubic or hexagonal structure is coupled to nanocrystal size. By selection of the starting materials and tuning of the synthesis conditions, the MS-derived SiC can be isolated as nanoparticles (NPs), porous SiC/C composites with small primary crystals (2 - 4 nm), and as nanospheres. We also show that the SiC nanostructures are active for electrochemical hydrogen evolution reaction, and the activity can be remarkably improved by loading Pt (NPs) onto the structure.

highly crystalline NCs based upon strongly covalent bonds, including Si, Ge and metal borides.12-14

1. INTRODUCTION There is a steady pursuit of a green and versatile strategy for the synthesis of nanoscale materials. Using bottom-up approaches based on solution chemistry, a variety of nanomaterials with different bonding nature have been produced, including metallic, ionic, covalent and mixed bonded systems.1,2 The transformation from precursors dissolved in solvent to nanocrystals (NCs) is kinetically related to the bonding strength of the target material. Generally, nanomaterials with predominantly metallic bonding (such as noble metals), or ionic bonding (such as fluorides and oxides) can be easily accessed under mild synthetic conditions with good crystallinity. On the contrary, wet-chemistry route derived strong covalent bonded solids, such as borides, carbide and nitrides, are always small in crystal size, poorly crystalline or even amorphous.3-7 To overcome the energy barrier of crystal nucleation, a much higher temperature is usually required, which however exceeds the operation window for most of the organic solvents under ambient pressure. Hydrothermal or solvothermal condition are therefore applied to avoid this temperature limitation, leading to successful growth of some metal carbides and borides nanomaterials with well-defined crystalline morphology under elevated temperatures.8-11 However, reactions at autogenous high pressure and high temperatures are difficult to work with. This situation stimulated the synthesis in inorganic molten salt (MS) media as green “solvent”, which offer an operational temperature window from 100 °C to over 1000 °C without developing dangerous pressures. This MS process has indeed already succeeded in the synthesis of

The MS route has shown its general applicability, while its potential has not been fully exploited, especially for microstructure and phase control of nanomaterials synthesis. We extend herein our exploration of nanomaterials synthesis using the MS route to silicon carbide (SiC), which is of practical importance as a material surviving harsh and oxidative conditions as well as in the optoelectronic field due to its semiconducting nature.15,16 Nanostructured SiC has been previously obtained using different routes, such as carbothermal reduction of silica,17,18 solvothermal synthesis,19 chemical vapor deposition,20 and pyrolysis of different organic precursors;21,22 the modulation of the texture for SiC at nanoscale was realized through the use of templates by nanocasting.23,24 On the other hand, although SiC is constructed solely from sp3 Si-C bonds, it exhibits striking structural diversity, being metastable in more than 250 polymorphs.25,26 The structures formed at nanoscale are in most cases limited to cubic 3C-SiC, and the factors that can drive the transition between different phases at nanoscale remain largely unclear. We show in the present work that the MS route could be a new solution for the scalable production of SiC nanomaterials in a controlled manner. We identified a crystal growth driven phase transition for nanoscale SiC, and confirmed the possibility of morphological transcription using templates in MS synthesis. We finally demonstrate that the MS-derived SiC nanostructures with high specific surface area can be used either as a catalyst or a catalyst

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support for the electrochemical hydrogen evolution reaction (HER).

2. EXPERIMENTAL SECTION Starting materials. Different types of silica (SiO2) were examined as precursors, including quartz (crystalline SiO2), silica nanoparticles (NPs) of 10 nm diameter, SBA15 and silica spheres. The preparation details of SBA-15 and silica sphere can be found elsewhere.27,28 Carbon sources examined in the present work are glucose (sp3 bonding), MS-carbon (mainly sp2 bonding) and graphite (sp2 bonding). MS-carbon is X-ray amorphous and was prepared by carbonization of glucose in a deep eutectic molten salt mixture (LiCl/KCl 45/55 in weight) at 700°C (details are described in supporting information (SI)). It has to be pointed out here that the MS carbon is not special in terms of composition and chemical characteristics; other types of amorphous carbons derived from wood or biomass result in similar SiC products. All chemicals involved in the present work were purchased from SigmaAldrich, and they were used without further treatment. Sample synthesis. Synthesis of SiC nanostructures with the MS route was performed along the same process, with only minor differences. In a typical process, silica, carbon and magnesium powders in a molar ratio of 1:2:3 were mixed with the salts mixture of LiCl/KCl at the weight ratio of reagent/salts = 1/5 and homogenized by ball milling. Afterwards, the mixture of the starting materials was loaded into an electric furnace and heated to the reaction temperature under continuous nitrogen flow. After being kept at the reaction temperature for 5 h, the system was cooled down to ambient temperature by switching off the furnace. The as-obtained black block of products which contained salts, SiC, residual carbon and MgO were immersed into sufficient amount of 1M HCl solution and then water to dissolve MgO and remove the salts. The insoluble solid remnant was collected by centrifugation and then dried in vacuum at ambient temperature for 48 h. The remaining carbon in the products was removed by heating the sample in air at 600 °C for 1 h. Finally, the residual SiO2 in the samples was removed by washing with 4M ammonium bifluoride (NH4F⋅HF) solution and drying in vacuum at ambient temperature. In case of D(+)-glucose monohydrate as carbon source, the ratio for the raw materials was adjusted to SiO2:carbon:Mg=1:3:8, considering that additional Mg is consumed by water released from glucose. In addition, oxidative carbon removal for the samples obtained in this route cannot be performed because the SiC particles form with a small crystal size (< 5 nm) and are highly reactive in air at elevated temperatures (see Figure S1). The residual carbon contained in the samples can be reduced by washing with H2O2 solution (30%) at ambient temperature, while complete removal of carbon from these samples is still not possible. The yields of SiC are around 75% and 60% for processes using carbon and glucose as starting materials, respectively. Structural characterization. The crystal structure as well as the crystallinity of the samples was examined by

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powder X-ray diffraction (XRD) with a Bruker D8 Advance diffractometer equipped with Cu-Kα radiation. The crystal size of the SiC was estimated using Scherrer equation: β = Kλ/Lcosθ, where β is the spectral width and K=0.89 is the Scherrer constant.29 The observed peaks in the XRD patterns (for sample prepared from MS carbon) were indexed and assigned to either 3C-SiC or 2H-SiC. The crystal structures were visualized using VESTA (Figure 1a).30 Based on the structural model of 3C-SiC and 2HSiC, Rietveld refinement for the XRD patterns was then performed with the EXPGUI-GSAS program to quantify phase fractions and determine precisely the lattice constants for nanoscale SiC.31 Peak profile and the background were described with the pseudo-Voigt function, and the shifted Chebyschev polynomial, respectively. The weight fractions of different phase were calculated by the software according to: Wp=(SZMV)p/∑(SZMV)i, where S is the scale factor which is determined by refinement, Z is the number of formula per unit cell, M is molecular weight, and V is volume of the unite cell. Morphological observations for the samples were made with scanning electron microscopy (SEM, LEO 1550) and transmission electron microscopy (TEM, Zeiss EM 912 operating at 120 kV). Further structural characterization was carried out using high resolution TEM (HR-TEM) with a JEOL 2011 system equipped with a LaB6 cathode at 200 kV. To quantitatively distinguish the structural difference between different samples, nitrogen absorption was measured at 77 K using a Quadrasorb Adsorption Instrument (Quantachrome Instruments). The surface area (SA) was calculated based on the multi-point BrunauerEmmett-Teller (BET) model, and the pore volume and pore size distribution were extracted from the analysis of the isotherms with the Barret-Joyner-Halenda (BJH) model. Modification with Pt and Electrochemical measurements. To perform the loading of Pt NPs, 640 μL Na2PtCl4 solution (2 mM) was added to a 20 mL water dispersion containing SiC NPs (5 mg) with continuous stirring (The nominal Pt loading is 5 wt.% for all the examined samples). After 60 min, NaBH4 solution (5 eq.) was added drop-wise to the mixture to reduce Na2PtCl4 to Pt NPs. Afterwards, the Pt loaded sample was isolated from the solution through repeated centrifugation and washing for 3 times. The obtained solid products were dried in vacuum at 80 °C for 12 h. All the electrochemical measurements were performed in a Versa STAT potentiostat system (Princeton Applied Research) using Pt wire and saturated calomel electrode (SCE) as the counter and the reference electrodes, respectively. To prepare the working electrode, 5 mg of the catalyst powders were dispersed in 200 μL ethanol under mild ultrasonic agitation for 1 h to form a homogeneous dispersion. The dispersion (2 μL) was then slowly drop-casted on to a glassy carbon (GC) electrode (diameter: 3 mm). The loading of the catalyst on the surface of GC electrode was around 0.71 mg/cm2 for each test. After sufficient drying in air, the catalyst thin film was covered with 1 μL of 0.1 wt% Nafion solution (in 1-propanol, Sigma-Aldrich)

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Chemistry of Materials

and further dried in air before measurement. The HER activity for all the samples was measured at weak acidic condition in a 0.5 M Na2SO4 solution (pH = 6.0), as described in a recent report.32 Before the measurement, the electrolyte solution was purged with nitrogen at ambient conditions for over 30 min.

3. RESULTS AND DISCUSSION Cubic and hexagonal SiC from MS synthesis. In the present experiments, the deep eutectic salt mixture LiCl/KCl (45/55 by weight) with a melting point of 353 °C was selected as the solvent which provides an inert liquid environment from its melting point to over 1000 °C without drastic evaporation losses. The synthesis of SiC is based on magnesiothermic reduction of silica in the presence of a carbon precursor through the following overall reaction: SiO2+2Mg+C→SiC+2MgO. We first examined a range of silica and carbon precursors for the synthesis of SiC in MS. Our initial results indicate that the synthesis of SiC under mild condition relies on the reactivity between the silica and carbon sources. Crystalline starting materials, including quartz and graphite, could be excluded as they only lead to very limited conversion to SiC ( 20 nm). Microstructure control for nanoscale SiC by MS synthesis. Microstructural observations for SiC nanostructures were made with electron microscopy (EM) techniques, as shown in Figure 2. Herein, we start our interpretation from the sample synthesized from silica NPs at 750 °C (SiC-750C). From SEM and TEM observations (Figure 2a-2c), the sample is composed of homogeneous but slightly aggregated NPs with an average size of around 10-20 nm. Note that aggregation is a commonplace for NPs without added stabilizer, which is the case here. As indicated by HR-TEM image shown in Figure 2c,

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the sample also exhibits clear lattice fringes, and welldefined ring (in ED pattern) structures of the (111), (220) and (311) planes for cubic SiC are observed, pointing to a polycrystalline nature.39 A higher reaction temperature leads to significant crystal growth, and therefore larger crystals and agglomerates are observed (Figure S6). The transformation from cubic to hexagonal structure starting around 800 °C as observed by XRD is also supported by the weak diffraction rings in ED pattern that appear near the (111) ring of cubic SiC (Figure S6).

Figure 2. Morphological control for SiC nanostructures. (a-c) and (d-f) are structural observations for SiC-750C (from silica NPs and MS-carbon) and SiC-750G (from silica NPs and glucose), respectively, by SEM (a, d), TEM (b, e) and HR-TEM (c, f). The insets in (b) and (e) are ED patterns, indicating the presence of cubic SiC. (g) and (h) are SEM and TEM (insets) images for samples of only SiC-750C/T and SiC-750G/T, respectively, which were synthesized using silica sphere as a hard template and the two different carbon sources: MScarbon (g), and glucose (h). For SiC-750G/T the residual cannot be removed, and hence a sphere-in-matrix structure is observed.

A completely different microstructure is obtained by using glucose as the carbon precursor instead of MScarbon (Figure 2d-2f). Since the sample contains residual carbon which is not possible to remove by oxidation in air, the structures are actually a homogeneous nanocomposite of SiC@C. TEM images for SiC-750G reveal an extended carbon network with dispersed tiny SiC particles, which are hardly visible. The presence of SiC particles with a cubic crystal structure is proved by the three characteristic rings for 3C-SiC in the electron diffraction (ED) pattern. The ED pattern again implies extremely poor crystallinity and small crystal size. The small crystallites dispersed within the carbon matrix as detected with HRTEM (Figure 2f) are around 2-4 nm in size, which agrees

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Further quantitative information on the microstructure of the MS-derived SiC nanomaterials is provided by nitrogen sorption measurements. The adsorption isotherms and pore size distributions are shown in Figure 3 and Figures S9, S10. The use of glucose as the carbon precursor leads to much higher surface area (SA) of the product. For SiC-750C synthesized from MS-carbon, the SA and the pore volume are 204 m2/g and 0.71 cm3/g, respectively (Table 1). The results are of the order expected for rather freely accessible primary stacked NPs and go well with pore size distributions (Figure 3b). In contrast, the SiC nanostructures produced using glucose as the carbon sources show unexpectedly high SAs of 899 m2/g for SiC750G, and 1241 m2/g for SiC-750G/T. Considering the presence of around 25wt.% of residual carbon, we can still infer that the 3C-SiC NCs incorporated within the carbon network contribute to a large fraction of the observed SA, and that these particles are nicely accessible to the gas. The SA of the SiC/C hybrid developed in the MS is comparable to that of the porous SiC synthesized by nanocasting using templates or direct decomposition of organo-silanes.23,40-43 In the sorption curves of both SiC-750G and SiC-750G/T, a rapid uptake at low pressure is observed, indicating the presence of microporosity (pore

a 3

-1

Adsorbed volume @ STP (cm g )

800

600

SiC-750G/T 400

SiC-750G 200

SiC-750C

0 0.0

0.2

0.4

0.6

0.8

1.0

Pressure (P/P0)

b 1.5

0.08

-1

-1

0.06 1.0

dV(r) (cm nm g )

0.04

3

The use of templates allows further manipulation of the nanostructure. To realize structure transcription towards SiC in MS, we considered the use of different silica templates. In the MS system, it is often the case that the template is broken by the strain exerted by the volume change during melting and solidification of the salts. Even the versatile yet “hard” SBA-15 did not survive the MSthawing/freezing, resulting in pulverized particulate products (Figure S7). We therefore employed solid silica spheres of 150 nm in size as a stronger template for the structural transcription in MS. The EM images for SiC750C/T (T stands for Silica sphere template) show that the spherical morphology is preserved to a large extent after synthesis, while the structure becomes porous as a result of volume shrinkage from SiO2 to SiC coupled to magnesiothermic reduction (Figure 2g, 2h). The sample synthesized from silica spheres at 750 °C is made up dominantly of 3C-SiC, and it does not differ in phase composition from the samples prepared from the much smaller silica NPs under the same condition. Interestingly, hollow SiC spheres embedded in a carbon matrix were obtained using silica sphere as template and glucose as carbon precursor (SiC-750G/T, see Figure 2h). The spherical morphology of the template is preserved in MS, irrespective of the different carbon precursors (amorphous carbon or glucose). This fact has certain implications for the understanding of the formation mechanisms of SiC in the current system explained below.

size < 2 nm). The slight hysteresis at 0.5 P/P0 on the other hand suggests the presence of mesoporosity for both samples. The observed microporosity may correlate with the activation of carbon along the reaction: SiO2+2C→Si+2CO, which possibly generates pores in both of the resulting SiC and the remaining carbon.

-1

with the estimation based on peak width of the XRD pattern. The C/Si ratio determined by energy dispersive Xray spectroscopy (EDX) is 68/32, suggesting that the residual carbon phase is 25% by weight. These observations suggest that the carbon structure generated in-situ in the MS from glucose strongly confines the growth of the dissolved SiC crystals at elevated temperatures.

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Cumulative Pore Volume (cm g )

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Volume SiC-750G/T

0.5

dV(r) 0.02

SiC-750G SiC-750C 0.0

0.00 1

10

100

1000

Pore diameter (nm)

Figure 3. Microstructure characterizations by nitrogen sorption. (a) Representative absorption/desorption isotherms measured at 77 K for samples synthesized from different carbon and silica precursors. (b) The corresponding pore size distributions.

Table 1. Specific surface areas, pore fractions and pore sizes extracted from the nitrogen sorption isotherms for samples synthesized from different silica and carbon precursors.

Sample name

Surface area 2

Pore volume 3

(m /g)

(cm /g)

SiC-750C

204

0.75

SiC-850C

77

0.19

SiC-750G

899

0.89

SiC-850G

686

0.59

SiC-750C/T

191

0.51

SiC-750G/T

1241

1.32

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Figure 4. Schematic illustration for synthesis of SiC in MS from (a) silica NPs and carbon, (b) silica NPs and glucose. The process is divided into three stages. The major difference is that in (b) carbon matrix generated by carbonization of glucose traps most of the as-formed particles of intermediate products (such as Si) such that the growth in crystal size is hindered, while in (b) crystal growth occurs easily in free space through aggregation or precipitation.

Based on these observations, a few comments can be made regarding the differences in mechanism of SiC formation involving glucose and porous amorphous MScarbon. In case of silica NPs and amorphous carbon as starting materials, the formation of SiC by the reaction of carbon and in-situ reduced Si (most probably as insitu generated atoms or clusters) starts at a temperature below 700 °C,14 and the growth of SiC NCs proceeds with increasing temperature almost in free space without any hindrance. Since it was observed that the morphology of a bigger silica sphere can be preserved while carbon structures are completed disintegrated within the MS synthesis, the formation of SiC indeed nucleates from the etched, but still not completely converted silica particles and proceeds to the interior. The process is different when glucose is employed as a carbon source. Carbonization of glucose starts at a low temperature (